How Often Are Visibility AND Temperature Both Good? We Counted, Across 30,000 Measurements
2026-03-09
Every diver knows the trade-off by feel: the clearest water is rarely the warmest. But how often do both line up? We counted — no folklore, just ~30,000 observations where visibility and temperature were recorded together. Our "jackpot day" is defined as 15m+ visibility AND 22°C+ water (comfortable in a 5mm, with real sightlines).
The annual ranking: southern islands dominate — with a twist coming
| Rank | Site | Annual jackpot rate | Measurements |
|---|---|---|---|
| 1 | Yonaguni | 99% | 4,665 |
| 2 | Kerama | 76% | 1,350 |
| 3 | Amami-Ōshima | 68% | 833 |
| 4 | Hachijojima | 38% | 448 |
| 5 | Mikomoto | 23% | 2,243 |
| 6 | Kashiwajima | 20% | 1,125 |
| 7 | Akinohama (Izu Oshima) | 16% | 1,307 |
| 8 | Koganezaki | 15% | 1,345 |
| 9 | Kushimoto | 13% | 2,885 |
| 10 | Izu Ocean Park | 10% | 3,241 |
Year-round, the southern islands win exactly as you'd expect (the winter-temperature side of this story). But the table hides its best secret.
Pick the right month, and the mainland becomes a southern island
Split the mainland sites by month and the picture flips:
| Site | Best month | Jackpot rate that month | Annual rate |
|---|---|---|---|
| Akinohama (Izu Oshima) | October | 74% (September: 72%) | 16% |
| Kashiwajima | November | 51% | 20% |
| Mikomoto | September | 41% (October: 40%) | 23% |
| Izu Ocean Park | October | 37% | 10% |
| Kushimoto | August | 35% (typhoon dip to 20% in Sep, second peak 30% in Oct) | 13% |
Akinohama's 74% in October is statistically the same as Kerama's 76% annual rate. You don't need a flight — pick the month, and southern-island odds sit a jetfoil ride from Tokyo. That's the headline finding of this dataset.
The pattern is clean: the mainland's jackpot window is September–November, when summer's warm water lingers while summer's turbidity drains away (why the warmest month is September, not August). And in December the Izu sites collapse to ~0% — visibility is actually heading toward its yearly best, but the water drops below 22°C. "Cold but clear" is real; it just isn't a jackpot day.
How to use this
- Heading south? Don't overthink timing (Yonaguni 99%, Kerama 76%) — watch typhoons and sea state instead
- On the mainland, aim for September–November — Akinohama in Sep–Oct is the strongest window on Honshu's doorstep
- Right before the trip, check your site's latest measured visibility and temperature in the archive (last week's reading is your best forecast)
Note: Rates are based on days shops actually dived, so blown-out days aren't in the denominator (read it as "of the days you can get in, how many hit the jackpot"). For suit choice, see the temperature guide.
Water temperatures at a glance
Data sources
- Visibility & temperature measurements: this site's database (2006–2026; only observations recording both, per-site counts in the tables), browsable in the archive
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